Hi!

I would like to design an electronic system that plugs between a
phone/fax/etc. and the phone line.
This system should just change some digits of the number being dialed
(usually only the first digit).
The digits should only be changed at specific time ranges and specific days
of the week.

Therefore this project needs the following parts:
- Real time clock/calendar
- DTMF decoder
- DTMF generator
- Analog circuitry (mainly for initially disabling the DTMF tones from the
phone and then connecting
 the phone to the phone line)

That first looks like a simple project. However, I would like to use as few
parts as possible. My question is:
is is realistic to have the clock, DTMF decoding and encoding tasks all
done by a PIC microcontroler?
I already spent several hours searching on the Internet and found solutions
for DTMF tone generating.
Doing a clock/calendar with timer/software doesn't look very complicated
(at first glance). But DTMF
decoding looks a lot more complicated (A/D converter + DFT?). A Fourrier
transform must obviously
eat up a lot of PIC ressources (execution time and code memory). Maybe
someone has found a trick
for detecting DTMF tones in a simpler way. Who knows?

Concerning the analog circuitry, the more I think about it, the more
complicated it looks to me. I see
two different approaches for changing the first digit of the dialed number:
1 - The phone is permanently connected to the phone line. But I need to
find a way to intercept (at least
the first two) DTMF tones. Then the PIC has to dial the new changed
digit(s) while eventualy getting additional
tones from the phone.
2 - The phone is temporarily disconnected from the phone line (with a
relay). But here, how can the dial
tone, busy tone, operator voice (etc..) be heard by the person (or fax,
modem) who makes the call.

Actually I have no solution to the problems above. I would apreciate any
good ideas from experienced
PIC developers. I haven't design anything since the 68705 and 8748
microcontrolers were very popular!

Louis Grau.
France